1. Field of the Invention
The present invention relates to a rewritable optical recording medium capable of information-recording, reproducing and erasing according to a surface-recording/reproducing system by using a flying optical head.
2. Discussion of Background
An optical recording medium is a portable recording medium permitting large volume/high density recording, and there have been rapidly increasing demands of rewritable media for recording dynamic images and so on as large volume preserving files for computers in current fashion of multimedia.
In an optical recording medium, a guide groove (hereinbelow, referred to as a groove) for trucking, by the aid of a servo, laser beams to a position for recording/reproducing is generally formed in a physical shape in the substrate, and information is recorded in a convex portion (hereinbelow, referred to as a land) between grooves in the substrate.
Accordingly, as the land is made larger, it is possible to increase the width of marks for recording; the intensity of reproducing signals can be increased, and the quality of reproducing signals can be improved.
In order to perform high density recording, however, it is necessary to increase an information quantity per recording surface area by reducing the track pitch as the distance between grooves.
For example, in a commercialized magneto-optical recording medium of 3.5 inch diameter, the track pitch has been reduced to 1.6 μm in a recording medium having a recording capacity of 128 MB, 1.4 μm in that of 230 MB and 1.1 μm in that of 640 MB.
On the other hand, the recording density is limited by a laser beam spot size (˜λ/NA) which is determined by a laser wavelength (λ) from a light source and a numerical aperture (NA) of an objective lens.
For example, in the recording/reproducing device for the above-mentioned 640 MB magneto-optical recording medium, the wavelength is 680 nm and NA is 0.55, and therefore, the laser beam spot size is about 1240 nm.
As means for reducing the laser beam spot size to achieve high density recording, a so-called near-field optical recording wherein recording/reproducing is conducted by bringing the optical head close to the recording layer, has been noted (e.g., (Appl. Phys. Lett.), vol. 68, p. 141 (1996)). In this recording method, a solid immersion lens (hereinbelow, referred to as “SIL”) head is used, and recording/reproducing of super-high recording density can be realized by increasing an effective numerical aperture with use of SIL to thereby reduce the laser beam spot size.
For example, in the near-field optical recording using SIL having a wavelength of 650 nm and an effective NA of 1.4, the laser beam spot size is about 460 nm, which is about 37% of the laser beam spot size used in the before-mentioned conventional recording/reproducing device for 640 MB magneto-optical recording medium.
In this surface recording/reproducing method, it is necessary to bring the optical head close to the recording medium, and accordingly, laser beams are not irradiated to the recording layer through the substrate as in the conventional optical recording medium, but a method for irradiating directly laser beams to the recording layer without being passed through the substrate is used.
Namely, the structure of the recording layer in the conventional optical recording medium comprises generally substrate/first protective layer/recording layer/second protective layer/reflective layer. On the other hand, in the near-field optical recording, it has an layer structure, contrary to the above, of substrate/reflective layer/recording layer/protective layer so that recording/reproducing is carried out by irradiating laser beams from a layer surface side.
In this case, use of a flying slider head is proposed to bring the optical head close to the recording layer.
Generally, when a track pitch P is in a size as much as an optical spot, a groove as a guide groove functions as a diffraction grating, and the intensity distribution of the beam spot is changed by the effect of interference due to a track deviation in a region where the 0-order diffraction light and the first-order diffraction light overlap, whereby a tracking error signal can be detected.
The intensity of this signal is determined by the numerical aperture NA of an objective lens, a track pitch and the wavelength λ of laser beams, and it is known that the maximum tracking error signal is provided when the depth of a groove is λ/8 n (n: the refractive index of a substrate through which laser passes).
However, there were the following problems in the near-field optical recording system. Since the signal was obtained by a coupling effect by the optical head at neighboring position and the outermost surface of the disk in addition to the known diffraction effect, the signal largely varied depending on a flying height of the optical head. Further, with respect to designing of the depth of grooves, the optimum depth for the near-field optical recording could not be obtained by the conventional designing technique.
Further, the optical recording medium has a header area in which a header comprising convex bumps or concave pits having a format information is formed and a data area having a land and a groove used for tracking the optical head and recording/reproducing of data. The optical recording medium had such a problem that the flying height of the head in recording/reproducing fluctuated because the flying height of the head flying above the header area was different from the flying height of the head flying above the data area. In description of the present invention, the substrate in which a land and a groove are formed is referred to as the substrate having a land/groove structure, and the portion of a land and the portion of a groove in the data area are referred respectively to as the land portion and the groove portion.
Further, in the surface optical recording system, since it is necessary to bring the optical head close to the medium, a so-called head crush wherein the flying slider head hits the medium is apt to occur. Accordingly, if the surface of the recording medium has not sufficient lubricating properties, a slight change in the flying height of the flying optical head from the recording medium will cause the contact of the flying optical head to the recording medium, with the result that the head and the recording medium are broken. Further, since the laser spot diameter is small in recording/reproducing of the recording medium, noises of recording/reproducing signals become large if the surface roughness of the recording medium is large, whereby sufficient SNR can not be obtained and there is a trouble in recording/reproducing.
Thus, in the near-field recording medium according to the recording/reproducing system for which the flying slider head is used, it was difficult to keep the flying height of the flying optical head constant in the entire recording/reproducing region and to obtain an excellent recording/reproducing signal uniformly. Further, there was the problem that the flying optical head contacted the recording medium to thereby cause the breakage of the head and the recording medium. Thus, it was difficult to obtain the excellent recording/reproducing signal in view of reliability and durability.